There has been widely known a technology in which an air fuel ratio sensor for generating an air fuel ratio signal corresponding to the oxygen concentration of an exhaust gas is installed on an exhaust system of a compression ignition internal combustion engine, so that the air fuel ratio of the exhaust gas is controlled by adjusting the amount of fuel to be injected from a fuel injection valve and the fuel injection timing thereof, the amount of fuel to be injected directly into the exhaust system from a fuel addition valve or the like.
However, in the air fuel ratio sensor, a protector for the sensor, etc., can be clogged due to particulate materials or the like contained in the exhaust gas, during which the function of the air fuel ratio sensor might be degraded or deteriorated because of a change in the property of electrodes of the sensor due to the thermal deterioration thereof. If the air fuel ratio of the exhaust gas is controlled based on the air fuel ratio signal from the air fuel ratio sensor thus degraded, there will be fear that the actual air fuel ratio of the exhaust gas might become far away from a target air fuel ratio exhaust gas, thereby deteriorating the exhaust emission. Accordingly, as a technique to determine the deterioration of the air fuel ratio sensor, there has been disclosed one in which when the time required of the air fuel ratio sensor from the detection of a predetermined rich-side air fuel ratio until the detection of a predetermined lean-side air fuel ratio or the time required of the air fuel ratio sensor from the detection of a predetermined lean-side air fuel ratio until the detection of a predetermined rich-side air fuel ratio is longer than a reference time, a determination is made that the air fuel ratio sensor is degraded (see, for example, Japanese patent application laid-open No. H10-18886).
In addition, there has also been disclosed a technique in which the deterioration of an air fuel ratio sensor is determined based on the response of the air fuel ratio of an exhaust gas detected by the air fuel ratio sensor, and a reference value for deterioration determination is changed according to the cases where the air fuel ratio of the exhaust gas is at a lean side, or in the vicinity of a stoichiometric air fuel ratio, or at a rich side (see, for example, Japanese patent application laid-open No. H10-280991).